This invention relates to radio telecommunication systems. More particularly, and not by way of limitation, the invention is directed to a control system and method for controlling a spread-spectrum radio receiver such as a generalized Rake (G-Rake) receiver or a chip equalizer based on a frequency offset of a received signal.
RAKE receivers are well known in the communication arts and find widespread use in Code Division Multiple Access (CDMA) systems, such as in IS-95, IS-2000 (cdma2000), and Wideband CDMA (WCDMA) wireless communication networks. The name derives from the rake-like appearance of such receivers, wherein multiple, parallel receiver fingers are used to receive multiple signal images in a received multipath signal. By coherently combining the finger outputs in a RAKE combiner, the conventional RAKE receiver can use multipath reception to improve the Signal-to-Noise Ratio (SNR) of the received multipath signal.
However, as is known to those skilled in the art, the conventional RAKE receiver is optimal only in certain limited circumstances. For example, self-interference and multi-user access interference both degrade the performance of a conventional RAKE receiver. To that end, the assignee of the instant application has made application for one or more patents relating to the use of a “generalized” RAKE receiver architecture, wherein receiver performance is improved by increasing the sophistication of combining weight generation.
In the generalized RAKE architecture, the combining weight calculations consider correlations of one or more signal impairments across RAKE fingers. For example, a generalized RAKE receiver may track noise correlations across those fingers. Generalized RAKE receivers also may include a comparatively larger number of fingers such that extra fingers may be positioned off of the signal path delays. A generalized RAKE receiver may gain performance improvements by shifting these extra fingers to optimize the SNR of the received signal. Correlations of signal impairments can also be used in SNR estimating often referred to as signal-to-interference ratio (SIR) estimation. SIR estimation is used in power control as well as in monitoring link quality and rate adaptation.
Note that the G-Rake receiver builds on the Rake receiver to improve receiver performance. That is, the received signal is despread by correlators matched to particular signal delays, and then the despread values are weighted and combined. Since the despreading and combining operations are linear, the order may be interchanged without losing performance. A receiver structure known as a chip equalizer embodies such an approach. Here, weights are applied at the chip level to combine/equalize the received signal. The combined/equalized signal is then despread by a single correlator. The chip equalizer weights are also a function of the impairment covariance matrix.
Thus, both the G-Rake and chip equalizer receivers suppresses interference through combining weights that account for the overall interference plus noise environment. What follows is described in terms of a G-Rake receiver, but equivalent operations exist for a chip equalizer. Estimates of the received traffic symbols are obtained via:z=wHx,   (1)where w is a vector of G-Rake combining weights, x is a vector of despread traffic symbols, and z is the traffic symbol estimate.
The G-Rake combining weight vector is obtained from:w=Ru−1h,   (2)where Rij is the impairments covariance matrix (own-cell interference plus other-cell interference plus noise) and h is a vector of net channel coefficients, including the contributions of the transmit filter, radio channel, and receive filter.
The impairments covariance matrix is given by:
                              R          u                =                                                            E                c                            ⁡                              (                0                )                                      ⁢                                          R                0                            ⁡                              (                                                      g                    ~                                    0                                )                                              +                                    ∑                              j                =                1                            J                        ⁢                                                  ⁢                                                            E                  c                                ⁡                                  (                  j                  )                                            ⁢                                                R                  j                                ⁡                                  (                                                            g                      ~                                        j                                    )                                                              +                                    N              0                        ⁢                                          R                n                            .                                                          (        3        )            
Here, E(j) is the energy per chip of base station j; the R0 term represents own-cell interference; the summation of terms over j represents other-cell interference; and N0Rn is the contribution of white noise passed through the receive filter. Note that equation (3) has been written to clearly illustrate the dependence of the impairments covariance matrix on the estimated medium channel coefficients {tilde over (g)}. The medium coefficients needed for (3) are obtained from estimates of net cofficients via:
                                          g            ~                    j                =                                            [                                                                                                                  R                        p                                            ⁡                                              (                                                                                                            τ                              j                                                        ⁡                                                          (                              0                              )                                                                                -                                                                                    τ                              j                                                        ⁡                                                          (                              0                              )                                                                                                      )                                                                                                                                                R                        p                                            ⁡                                              (                                                                                                            τ                              j                                                        ⁡                                                          (                              0                              )                                                                                -                                                                                    τ                              j                                                        ⁡                                                          (                              1                              )                                                                                                      )                                                                                                  …                                                                                                      R                        p                                            ⁡                                              (                                                                                                            τ                              j                                                        ⁡                                                          (                              0                              )                                                                                -                                                                                    τ                              j                                                        ⁡                                                          (                                                                                                L                                  j                                                                -                                1                                                            )                                                                                                      )                                                                                                                                                                                R                        p                                            ⁡                                              (                                                                                                            τ                              j                                                        ⁡                                                          (                              1                              )                                                                                -                                                                                    τ                              j                                                        ⁡                                                          (                              0                              )                                                                                                      )                                                                                                                                                R                        p                                            ⁡                                              (                                                                                                            τ                              j                                                        ⁡                                                          (                              1                              )                                                                                -                                                                                    τ                              j                                                        ⁡                                                          (                              1                              )                                                                                                      )                                                                                                  …                                                                                                      R                        p                                            ⁡                                              (                                                                                                            τ                              j                                                        ⁡                                                          (                              1                              )                                                                                -                                                                                    τ                              j                                                        ⁡                                                          (                                                                                                L                                  j                                                                -                                1                                                            )                                                                                                      )                                                                                                                                  ⋮                                                        ⋮                                                        ⋱                                                                                                                                                                                                                                        R                        p                                            (                                                                                                    τ                            j                                                    ⁡                                                      (                                                                                          L                                j                                                            -                              1                                                        )                                                                          -                                                                                                                                                R                        p                                            (                                                                                                    τ                            j                                                    ⁡                                                      (                                                                                          L                                j                                                            -                              1                                                        )                                                                          -                                                                                                  …                                                                                                      R                        p                                            (                                                                                                    τ                            j                                                    ⁡                                                      (                                                                                          L                                j                                                            -                              1                                                        )                                                                          -                                                                                                                                                                                                          τ                          j                                                ⁡                                                  (                          0                          )                                                                    )                                                                                                                                                    τ                          j                                                ⁡                                                  (                          1                          )                                                                    )                                                                                                                                                                                                                                                      τ                          j                                                ⁡                                                  (                                                                                    L                              j                                                        -                            1                                                    )                                                                    )                                                                                  ]                                      -              1                                ⁢                                                    h                ^                            j                        .                                              (        4        )            
Here, Rp(τ) is the pulse shape autocorrelation function, τj is a vector of path delays for base station j (Lj paths for base station j) and ĥ is the estimate of the net channel coefficients.
Mobile terminals may utilize a low-rate channel estimation approach in order to reduce receiver complexity. For example, a mobile terminal may estimate net channel coefficients utilizing:
                                                        h              ^                        j                    =                                    1              10                        ⁢                                          ∑                                  k                  =                  0                                9                            ⁢                                                          ⁢                                                                    x                    p                                    ⁡                                      (                    k                    )                                                  ⁢                                                      s                    *                                    ⁡                                      (                    k                    )                                                                                      ,                            (        5        )            where xp(k) is a vector of despread pilot symbols for the kth symbol in a slot, and s(k) is the kth pilot symbol in the slot. Note that this approach, referred to as the “slot average” estimate, produces one set of channel coefficients per slot.
The fundamental limitation of using low-rate channel estimation is that combining weights based on such estimates (see equations (2)-(4)) represent the average impairment scenario. When a mobile terminal is moving at low speed, the difference between the average and instantaneous impairment scenario is small, so the approach works well. However at moderate to high speed, the impairment scenario changes more rapidly, so there can be significant differences between the average and instantaneous impairment scenarios. This mismatch causes receiver performance to degrade, sometimes substantially.
What is needed in the art is a control system and method for producing unique combining weights in a spread-spectrum radio receiver which overcomes the shortcomings of the prior art. The present invention provides such a control system and method.